I am not sure how much hard evidence is behind the savanna hypothesis... but it seems that most scientist believe that an environmental change was necessary to drive evolutionary processes... when it comes to the question of why did some apes leave the forest, started to walk upright and think more intelligently... well, it is dificult to find all answers... I recall that my zoology professor had his own theory of why man started to walk upright. He said that there were mainly three reasons:
Our ancestors did not eat only fruits like the other apes, but also rotten meat from dead animals. We were not able to kill big animals, and therefore we had to eat what the lions and the other animals left. In order to find such spots as quickly as possible, we had to look into the sky to see the birds that started to make circles around the dead animals. When we saw that, we started to run to reach that spot. For this purpose it was advantageous if we became a better vision and standing upright was one solution. Interestingly, the humans are very formidable long distance runners. In fact, there is no animal that can run longer distances than humans. Other animals can run faster, but only over shorter distances. Humans are able to run over 60km without stopping. We have also the advantage of sweating glands, and this cools us down. And running upright has also the advantage that we are less exposed to the sun, and therefore get less heated.
It is quite intriguing, but many animals have more respect for the animals that are taller. In biology, size matters. It means more power and more strength. Many animals that walk on four feet, also walk on two feets from time to time. For example, bears and Gorillas. This is often a means for impressing the enemy. For example, when gorillas or chimps want to show up, they walk upright and they hit their chest with their hands. They also make strong sounds. The capability of making vowels might also be related to this need of making sounds to impress the others. Thus, walking upright was a means to demonstrate a kind of herarchy and create some respect for the others. Our pride is still very marked and this has been one of the key ingredients for our evolution... but at the same time, this is also the reason why we are violent and make so many wars to those who damage our pride or do not respect us. Interestingly, still until our days we want to become taller for social purposes and higher status. For example, women wear heals not because they are more comfortable than normal shoes, but because they make them taller. Thus, when you see a fashion show with women dressing extravagantly with high shoes, you can recognise that we still carry the inheritance of the early hominoids who wanted to become bigger for pride.
When you observe the chimps and other apes you can recognise many types of behaviour that are very similar to ours. The social structure is also similar and the interpersonal relations follow similar patterns. Many antropologist believe that looking at the apes of today can give us many hints to the early stages of our ancestors millions years ago. Greed is a very important emotion, because it drives us to obtain more, and therefore makes us more competitive. It helps us to survive the struggle for life. When you put a big pile of bannanas near a group of hungry chimps, many of them will start to fight for the banannas. Eventually, a big male starts to dominate the others. He takes strategic position to defend the pile of bannanas, but when there are many other competitors, he will not be able to defend the whole bunch of fruits. While he counters one competitor from the left and keeps an eye on other from the right, another might come from behind and take some banannas. One strategy that many animals follow when there are many competitors and only one central source of food, is to take a piece to another place where you can eat it quietly. If you take a small piece, you divide the attention, and because the big bunch is still at the original place, noboby will follow you. Thus, the strategy is to be quick, take a big bunch, and then run away as fast as possible. Now imagine that if you walk on four feets, then you have only your mouth to carry your cargo. But then, if you walk on two feet, your hands are free to carry more. Thus, those apes that want more bannanas, they use their hands to take as much as possible, run away to quiet place and eat their bannanas. This behaviour is also observed with the chimps of today, that normally walk on four feets, but when they want carry a lot, they walk on two feet. Therefore, greed and the desire for more possesions might also driven the evolution of hominoids to walk upright.
Diversity is Good!